Those expecting Germany to pave the way for fundamental EU reform are in for a "disappointment", Angela Merkel has told David Cameron.

She said a Europe without borders was one of the EU's "greatest achievements", but conceded: "What we are going to do, or fail to do, in reforming the political shape of Europe will determine its future.

"We need to reform the political shape of Europe in keeping with the times so that it may fulfil its promise of peace, freedom and prosperity also for future generations.

"Standstill may quickly turn into setback. Or to use a famous quotation of Winston Churchill, 'To be perfect is to change often'."

Mrs Merkel delivers her address in the Royal Gallery of the House of Lords

And she left no doubt that Berlin wanted the UK to remain a key member of the EU, adding: "We need a strong United Kingdom with a strong voice inside the European Union. If we have that, we will be able to make the necessary changes for the benefit of all."

Her latter comments will give some hope to the Prime Minister, who has spared no expense in welcoming the German Chancellor, who he considers a key ally as he fights for concessions from Europe ahead of a proposed referendum on Britain's membership.

He is hoping the visit will bolster his bid to negotiate a better deal for the UK in the European Union as he battles to win key EU opt-outs from his guest.

Mr Cameron aims to persuade Mrs Merkel to accept the need for treaty change which would allow him to table demands for the return of powers from Brussels to national parliaments.

David Cameron (R) listening to Mrs Merkel's speech

However, Mrs Merkel revealed she found herself "caught between the devil and the deep blue sea" on such European matters.

"Some expect my speech to pave the way for a fundamental reform of the European architecture which will satisfy all kinds of alleged or actual British wishes - I'm afraid they are in for a disappointment," she said.

"Others are expecting the exact opposite and are hoping that I will deliver the clear and simple message here in London that the rest of Europe is not prepared to pay almost any price to keep Britain in the European Union, I'm afraid these hopes will be dashed, too," she continued.

"I find myself caught between the devil and the deep blue sea. That, ladies and gentlemen, is not a pleasant position to be in."

She also talked about the controversial issue of immigration within the EU - the so-called free movement of people - and said that perhaps "mistakes" had been made.

David Cameron tweeted a picture of him with Angela Merkel

After the address the Prime Minister tweeted a picture of himself with the Chancellor in his Downing Street flat. He wrote: "I'm happy to welcome Angela Merkel to my Downing St flat, after her excellent address to Parliament."

Sky's Political Editor Adam Boulton said: "In one sense she was dampening down expectations that David Cameron is going to come back with a fundamental deal once he starts negotiations - but she certainly wasn't, as she put it herself, slamming the door in the face of the British government on at least reassessing Britain's relationship within the EU."

The German Chancellor was addressing both Houses of Parliament as part of a high-profile visit to London, which includes tea with the Queen later.

The occasion to speak to both MPs and peers is one usually reserved for British monarchs. Among those that have been granted the rare privilege is US President Barack Obama.

The pair at a joint news conference at Downing Street

Mrs Merkel thanked her audience in the Royal Gallery of the House of Lords for the "great honour" and the invitation which she said was an expression of the "close relations" between the two nations.

Formal talks with Mr Cameron took place over lunch at 10 Downing Street during which the PM said the pair had discussed ideas "on how to cut the excessive interference and meddling by European institutions in our national life", and the abuse of free movement through "benefits tourism".

"I want Britain to be a positive player in a reformed European Union ... Angela and I both want to see change in Europe," Mr Cameron said at a joint news conference at Number 10 with Mrs Merkel.

He added he was confident reforms would take place during the next few years.

Mrs Merkel indicated there was a "lot of common ground" the two countries shared over the issue of EU reform.

She immigration into "social systems" was just as much "a headache" for Germany as it is for Britain.